A locomotive has no payload capacity of its own, and its sole purpose is to move the train along the tracks.[2] In contrast, some trains have self-propelled payload-carrying vehicles. These are not normally considered locomotives, and may be referred to as multiple units, motor coaches or railcars. The use of these self-propelled vehicles is increasingly common for passenger trains, but rare for freight (see CargoSprinter). Vehicles which provide motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars.

Traditionally, locomotives pulled trains from the front. However, push-pull operation has become common, where the train may have a locomotive (or locomotives) at the front, at the rear, or at each end.

Prior to locomotives, the motive force for railroads had been generated by various lower-technology methods such as human power, horse power, gravity or stationary engines that drove cable systems.

The first successful locomotives were built by Cornish inventor Richard Trevithick. In 1804 his unnamed steam locomotive hauled a train along the tramway of the Penydarren ironworks, near Merthyr Tydfil in Wales. Although the locomotive hauled a train of 10 long tons (11.2 short tons; 10.2 t) of iron and 70 passengers in five wagons over nine miles (14 km), it was too heavy for the cast iron rails used at the time. The locomotive only ran three trips before it was abandoned. Trevithick built a series of locomotives after the Penydarren experiment, including one which ran at a colliery in Tyneside in northern England, where it was seen by the young George Stephenson.[3]

In 1814 George Stephenson, inspired by the early locomotives of Trevithick and Hedley persuaded the manager of the Killingworthcolliery where he worked to allow him to build a steam-powered machine. He built the Blücher, one of the first successful flanged-wheel adhesion locomotives. Stephenson played a pivotal role in the development and widespread adoption of steam locomotives. His designs improved on the work of the pioneers. In 1825 he built the Locomotion for the Stockton and Darlington Railway, north east England, which became the first public steam railway. In 1829 he built The Rocket which was entered in and won the Rainhill Trials. This success led to Stephenson establishing his company as the pre-eminent builder of steam locomotives used on railways in the United Kingdom, the United States and much of Europe.[5] The first inter city passenger railway, Liverpool and Manchester Railway, opened in 1830, making exclusive use of steam power for both passenger and freight trains.

Whether out of necessity to replace the locomotive due to failure, or for reason of needing to maintain the power unit, it is relatively easy to replace the locomotive with another, while not removing the entire train from service.

Maximum utilization of power cars

Separate locomotives facilitate movement of costly motive power assets as needed; thereby avoiding the expense associated with tied up or idle power resources.

Flexibility

Large locomotives can substitute for small locomotives when more power is required, for example, where grades are steeper. As needed, a locomotive can be used for either freight duties, or passenger service.

Obsolescence cycles

Separating motive power from payload-hauling cars enables replacement without affecting the other. To illustrate, locomotives might become obsolete when their associated cars did not, and vice versa.

Safety

In an accident, the locomotive may act as a buffer zone to the rest of the train. Depending on the obstacle encountered on the rail line, the heavier mass of a locomotive is less likely to deviate from its normal course. In the event of fire, it might be safer, for example, with diesel locomotives.

Noise

A single source of tractive power (i.e., motors in one place), is quieter than multiple operational power units, where one or more motors are located under every carriage. The noise problem is particularly noticeable in diesel multiple units.

Saves time

The motive power accompanies the cars to be hauled and consequently there is a saving in time.

Maintenance

Especially for steam locomotives but also for other types, maintenance facilities can be very dirty environments and it is advantageous not to have to take passenger accommodation into the same depot. This was one reason for the demise of the GWR steam railmotors.

There are several advantages of multiple unit (MU) trains compared to locomotives.

Energy efficiency

Multiple units are more energy efficient than locomotive-hauled trains and more nimble, especially on down grades, as much more of the train's weight (sometimes all of it) is placed on driven wheels, rather than suffering the dead weight of unpowered coaches.

No need to turn the locomotive

Many multiple units have cabs at both ends; therefore, the train may be reversed without uncoupling/re-coupling the locomotive, providing quicker turnaround times, reduced crew costs, and enhanced safety. In practice, the development of driving van trailers and cab cars has removed the need for locomotives to run-around, giving easy bi-directional operation and removing this MU advantage.

Reliability

Multiple unit trains have multiple engines, where the failure of one engine usually does not prevent the train from continuing on its journey. A locomotive drawn passenger train typically has only a single power unit; the failure of this single unit temporarily disables the train. However, as is often the case with locomotive hauled freight trains, some passenger trains utilize multiple locomotives, and are thus able to continue at reduced speed after the failure of one locomotive.

Locomotives may generate their power from fuel (wood, coal, petroleum or natural gas), or they may take power from an outside source of electricity. It is common to classify locomotives by their source of energy. The common ones include:

In the 19th century the first railway locomotives were powered by steam, usually generated by burning wood, coal, or oil. Because steam locomotives included one or more steam engines, they are sometimes referred to as "steam engines". The steam locomotive remained by far the most common type of locomotive until after World War II.[7]

The first steam locomotive was built by Richard Trevithick; it first ran on 21 February 1804, although it was some years before steam locomotive design became economically practical.[3] The first commercial use of a steam locomotive was Salamanca on the narrow gaugeMiddleton Railway in Leeds in 1812. In the USA, Mathias Baldwin started building stationary steam engines for commercial use and by 1830, opened his own workshop producing steam locomotives. Baldwin Locomotive Works became the world's largest by the early 1900s and built the most powerful steam locos in history. The locomotive Fairy Queen, built in 1855 runs between Delhi and Alwar in India and is the oldest steam locomotive in regular (albeit tourist-only) service in the world, and the oldest steam locomotive operating on a mainline.[8][9]

The all-time speed record for steam trains is held by an LNER Class A44-6-2Pacific locomotive of the LNER in the United Kingdom, number 4468 Mallard, which pulling six carriages (plus a dynamometer car) reached 126 mph (203 km/h) on a slight downhill gradient down Stoke Bank on 3 July 1938.[10] Aerodynamic passenger locomotives in Germany attained speeds very close to this and due to the difficulties of adequately balancing and lubricating the running gear, this is generally thought to be close to the practicable limit for a direct-coupled steam locomotive.[11]

Before the middle of the 20th century, electric and diesel-electric locomotives began replacing steam locomotives. Steam locomotives are less efficient than their more modern diesel and electric counterparts and require much greater manpower to operate and service.[12]British Rail figures showed the cost of crewing and fuelling a steam locomotive was some two and a half times that of diesel power, and the daily mileage achievable was far lower. As labour costs rose, particularly after the second world war, non-steam technologies became much more cost-efficient.[citation needed] By the end of the 1960s–1970s, most western countries had completely replaced steam locomotives in passenger service. Freight locomotives generally were replaced later. Other designs, such as locomotives powered by gas turbines, have been experimented with, but have seen little use, mainly due to high fuel costs.

By the end of the 20th century, almost the only steam power remaining in regular use in North America and Western European countries was on heritage railways. These were largely aimed at tourists and/or railroad hobbyists, known as 'railfans' or 'railway enthusiasts'. An exception are the narrow gauge lines in Germany, which form part of the public transport system, running to all-year-round timetables. These railways retain steam for all or part of their motive power. Steam locomotives remained in commercial use in parts of Mexico into the late 1970s. Steam locomotives were in regular use until 2004 in the People's Republic of China, where coal is a much more abundant resource than petroleum for diesel fuel. India switched over from steam-powered trains to electric and diesel-powered trains in the 1980s, except heritage trains. In some mountainous and high altitude rail lines, steam engines remain in use because they are less affected by reduced air pressure than diesel engines.[citation needed] Steam locomotives remained in routine passenger use in South Africa until the late 1990s, but are now reserved to tourist trains. In Zimbabwe steam locomotives are still used on shunting duties around Bulawayo and on some regular freight services.

As of 2006, DLM AG (Switzerland) continues to manufacture new steam locomotives.[13]

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Gasoline locomotives have been produced since the early 1900s.[14][15]

In 1916 Simplex petrol locomotives with 20-40 hp motors and 4-wheel mechanical transmission began to be used on 600mm gauge trench railways on the Western Front (World War I). One advantage of these machines was that they could operate closer to the front line than steam locomotives, the relative lack of exhaust helping to conceal their exact position.

Many were sold off as surplus at the end of hostilities, finding work on small industrial railways. Motor Rail continued to manufacture and develop the design, alongside diesel engined variants.

Experimental diesel-powered locomotives were first built just after World War I. In the 1940s, they began to displace steam power on American railroads. Following the end of World War II, diesel power began to appear on railroads in many countries. The significantly better economics of diesel operation triggered a dash to diesel power, a process known as Dieselization. By the late 1960s, few major railroads in North America, Europe and Oceania continued to operate steam locomotives, although significant numbers still existed outside these areas.

As is the case with any vehicle powered by an internal combustion engine, diesel locomotives require a power transmission system to couple the output of the prime mover to the driving wheels. In the early days of diesel railroad propulsion development, electric, hydraulic and mechanical power transmission systems were all employed with varying degrees of success. Of the three, electric transmission has proven the most popular, and although diesel-hydraulic locomotives have certain advantages and are continuously used in some European countries, most modern Diesel-powered locomotives are diesel-electric.

Diesel locomotives require considerably less maintenance than steam, with a corresponding reduction in the number of personnel needed to keep the fleet in service. The best steam locomotives spent an average of three to five days per month in the shop for routine maintenance and running repairs.[citation needed] Heavy overhauls were frequent, often involving removal of the boiler from the frame for major repairs. In contrast, a typical diesel locomotive requires no more than eight to ten hours of maintenance per month (maintenance intervals are 92 days or 184 days, depending upon a locomotive's age),[citation needed] and may run for decades between major overhauls.[citation needed]

Diesel units do not pollute as much as steam trains;[citation needed] modern units produce low levels of exhaust emissions. Diesel-electric locomotives are often fitted with "dynamic brakes" that use the traction motors as electrical generators during braking to assist in controlling the speed of a train on a descending grade. This technology is similar to regenerative braking used in hybrid cars; the key difference is dynamic braking does not store the generated power, instead it is routed to resistors then converted into waste heat.

A slug or drone locomotive is a non-powered unit attached to a diesel-electric locomotive to provide additional traction and braking capability. The slug has traction motors but no engine. Power is supplied by the attached locomotive (known as a 'mother'). At slow speeds, a diesel-electric prime mover can produce more power than its own traction motors can use; an installed slug increases the number of traction motors available, thereby using the created power more effectively.

Slugs are mainly used in rail yards for switching duties, in which case they are without a cab. Other slugs, designed for use on service trains, may be fitted with a cab, to enhance control, and also provide additional fuel storage for the mother locomotive. In recent years on service trains, conventional locomotives, remotely controlled from the lead locomotive configuration, have been used in place of slugs.

CP Rail used a prototype drone locomotive system called LOCOTROL, developed in the 1960s, which evolved into today's systems.

In 1894, a Hungarian engineer Kálmán Kandó developed high-voltage three phase alternating current motors and generators for electric locomotives.[19] His work on railway electrification was done at the Ganz electric works in Budapest. The first installation was on the Valtellina line, Italy, in 1902. Kandó was the first who recognised that an electric train system can only be successful if it can use the electricity from public networks. After realising that, he also provided the means to build such a rail network by inventing a rotary phase converter suitable for locomotive usage.

The electric locomotive is supplied externally with electric power, either through an overhead pickup or through a third rail. While the capital cost of electrifying track is high, electric trains and locomotives are capable of higher performance and lower operational costs than steam or diesel power.[citation needed] Electric locomotives, because they tend to be less technically complex than diesel-electric locomotives, are both easier and cheaper to maintain and have extremely long working lives, usually 40 to 50 years: the last unit of the Italian E626 class, introduced in 1928, was retired 71 years later, in 1999. There are many other examples of electric locomotives operating for more than half a century with minimal overhaul, and it is not unusual for electric locomotives to be operating close to their centenary.[citation needed] The Finnish State Railroad is planning to phase out the Soviet-manufactured VR Class Sr1 engines, operative since 1973, in 2024, at which time they will have been over fifty years in line service.

Some electric locomotives can also operate on battery power to enable short journeys or shunting on non-electrified lines or yards.[citation needed] Battery-powered locomotives are used in mines and other underground locations where diesel fumes or smoke would endanger crews, and where external electricity supplies cannot be used due to the danger of sparks igniting flammable gas.[citation needed] Battery locomotives are also used on many underground railways for maintenance operations, as they are required when operating in areas where the electricity supply has been temporarily disconnected.[citation needed]

Gas turbine locomotives are very powerful, and very loud. Union Pacific Railroad operated the largest fleet of gas turbine-electric locomotives in the world, and was the only railroad to use them for hauling freight in regular service. Most other GTELs were built for small passenger trains; only a few have seen any real success in that role.

After the 1973 oil crisis and subsequent rise in fuel costs, gas turbine locomotives became uneconomical to operate. Subsequently, many were taken out of service, making this type of locomotive rare.

In 2002 the first 3.6 tonne, 17 kW hydrogen (fuel cell) -powered mining locomotive was demonstrated in Val-d'Or, Quebec. In 2007 the educational mini-hydrail in Kaohsiung, Taiwan went into service. The Railpower GG20B finally is another example of a fuel cell-electric locomotive.

In the early 1950s, Dr. Lyle Borst of Utah University, was given funding by various US railroad line and manufacturers to study the feasibility of an electric-drive locomotive, in which an onboard atomic reactor produced the steam to generate the electricity. At that time, the dangers of atomic power were not fully understood; Borst believed the major stumbling block was the price of uranium. With the Borst atomic locomotive, the center section would have a 200-ton reactor chamber and steel walls 5 feet thick to prevent releases of radioactivity in case of accidents. He estimated a cost to manufacture atomic locomotives with 7000 h.p. engines at approximately $1,200,000 each.[23] Consequently, trains with onboard nuclear generators were generally deemed unfeasible due to prohibitive costs.

The three main categories of locomotives are often subdivided in their usage in rail transport operations. There are passenger locomotives, freight locomotives and switcher (or shunting) locomotives. These categories determine the locomotive's combination of physical size, starting tractive effort and maximum permitted speed. Freight locomotives are normally designed to deliver high starting tractive effort—needed to start trains that may weigh as much as 15,000 long tons (16,800 short tons; 15,241 t)—and deliver sustained high power, at the sacrifice of maximum speed. Passenger locomotives develop less starting tractive effort but are able to operate at the high speeds demanded by passenger schedules. Mixed traffic locomotives (US English: general purpose or road switcher locomotives) are built to provide elements of both requirements. They do not develop as much starting tractive effort as a freight unit but are able to haul heavier trains than a passenger engine.

Most steam locomotives are reciprocating units, in which the pistons are coupled to the drivers (driving wheels) by means of connecting rods, with no intervening gearbox. Therefore, the combination of starting tractive effort and maximum speed is greatly influenced by the diameter of the drivers. Steam locomotives intended for freight service generally have relatively small diameter drivers, whereas passenger models have large diameter drivers (as large as 84 inches or 2,134 millimetres in some cases).

Some locomotives are designed specifically to work steep grade railways, and feature extensive additional braking mechanisms and sometimes rack and pinion. Steam locomotives built for steep rack and pinion railways frequently have the boiler tilted relative to the wheels, so that the boiler remains roughly level on steep grades.

Train engine is the technical name for a locomotive attached to the front of a railway train to haul that train. Alternatively, where facilities exist for push-pull operation, the train engine might be attached to the rear of the train;

In the second half of the twentieth century remote control locomotives started to enter service in switching operations, being remotely controlled by an operator outside of the locomotive cab. The main benefit is one operator can control the loading of grain, coal, gravel, etc. into the cars. In addition, the same operator can move the train as needed. Thus, the locomotive is loaded or unloaded in about a third of the time.[citation needed]

Locomotives have been a subject for collectors' coins and medals. One of the most famous and recent ones is the 25 euro 150 Years Semmering Alpine Railway commemorative coin. The obverse shows two locomotives: a historical and a modern one. This represents the technical development in locomotive construction between the years 1854 and 2004. The upper half depicts the “Taurus”, a high performance locomotive. Below is shown the first functional Alpine locomotive, the Engerth; constructed by Wilhelm Freiherr von Engerth.